1. Field of the Invention
The invention relates to an electric power steering system.
2. Discussion of Background
Conventionally, most of motor control devices used in, for example, electric power steering systems (EPS) include malfunction detecting means for detecting occurrence of a malfunction when an energization failure occurs in one of a U-phase, a V-phase and a W-phase of a motor due to, for example, a break in a power supply line or a contact failure of a drive circuit. When occurrence of a malfunction is detected, generally, motor control is quickly stopped to provide failsafe protection.
In an EPS, however, the steering characteristics significantly change when the motor control is stopped as described above. That is, because assistance is no longer provided by the motor, larger steering operation force is required. US 2008/0203963 A1 describes an electric power steering system in which, even when a phase where an energization failure has occurred is detected as described above, motor control is continued with the use of the two phases other than the phase where the energization failure has occurred. In this way, it is possible to avoid an increase in a load on a driver caused by provision of the fail-safe protection, by continuing supply of assist force to a steering system.
However, in the electric power steering system in which the motor control is continued using the two phases, other than the phase where the energization failure has occurred, as energization phases, a short circuit between the phase where the energization failure has occurred and one of the other two phases may occur. If current flows through the short-circuited two phases, the motor may lock.